4.2.6.1 Laboratory data and the OH lifetime of greenhouse gases

Laboratory data on the rates of chemical reactions and photodissociation provide
a cornerstone for the chemical models used here. Subsequent to the SAR there
have been a number of updates to the recommended chemical rate databases of
the International Union of Pure and Applied Chemistry (IUPAC 1997a,b, 1999)
and the Jet Propulsion Laboratory (JPL) (DeMore et al., 1997; Sander et al.,
2000). The CTMs in the OxComp workshop generally used the JPL-1997 database
(JPL, 1997) with some updated rates similar to JPL-2000 (JPL, 2000). The most
significant changes or additions to the databases include: (i) revision of the
low temperature reaction rate coefficients for OH + NO2 leading to
enhancement of HOx and NOx abundances in the lower stratosphere
and upper troposphere; (ii) extension of the production of O(1D)
from O3 photodissociation to longer wavelengths resulting in enhanced
OH production in the upper troposphere; and (iii) identification of a new heterogeneous
reaction involving hydrolysis of BrONO2 which serves to enhance HOx
and suppress NOx in the lower stratosphere. These database improvements,
along with many other smaller refinements, do not change the overall understanding
of atmospheric chemical processes but do impact the modelled tropospheric OH
abundances and the magnitude of calculated O3 changes by as much
as 20% under certain conditions.

Reaction rate coefficients used in this chapter to calculate atmospheric lifetimes
for gases destroyed by tropospheric OH are from the 1997 and 2000 NASA/JPL evaluations
(DeMore et al., 1997; Sander et al., 2000) and from Orkin et al. (1999) for
HFE-356mff2. These rate coefficients are sensitive to atmospheric temperature
and can be ten times faster near the surface than in the upper troposphere.
The global mean abundance of OH cannot be directly measured, but a weighted
average of the OH sink for certain synthetic trace gases (whose budgets are
well established and whose total atmospheric sinks are essentially controlled
by OH) can be derived. The ratio of the atmospheric lifetimes against tropospheric
OH loss for a gas is scaled to that of CH3CCl3 by the
inverse ratio of their OH-reaction rate coefficients at an appropriate scaling
temperature. A new analysis of the modelled global OH distribution predicts
relatively greater abundances at mid-levels in the troposphere (where it is
colder) and results in a new scaling temperature for the rate coefficients of
272K (Spivakovsky et al., 2000), instead of 277K (Prather and Spivakovsky, 1990;
SAR). The atmospheric lifetimes reported in Table 4.1
use this approach, adopting an “OH lifetime” of 5.7 years for CH3CCl3
(Prinn et al., 1995; WMO, 1999). Stratospheric losses for all gases are taken
from published values (Ko et al., 1999; WMO, 1999) or calculated as 8% of the
tropospheric loss (with a minimum lifetime of 30 years). The only gases in Table
4.1 with surface losses are CH4 (a soil-sink lifetime of 160
years) and CH3CCl3 (an ocean-sink lifetime of 85 years).
The lifetime for nitrogen trifluoride (NF3) is taken from Molina
et al. (1995). These lifetimes agree with the recent compendium of Naik et al.
(2000).

Analysis of the CH3CCl3 burden and trend (Prinn et al.,
1995; Krol et al., 1998; Montzka et al., 2000) has provided a cornerstone of
our empirical derivations of the OH lifetimes of most gases. Quantification
of the “OH-lifetime” of CH3CCl3 has evolved over the past
decade. The SAR adopted a value of 5.9 ± 0.7 years in calculating the
lifetimes of the greenhouse gases. This range covered the updated analysis of
Prinn et al. (1995), 5.7 years, which was used in WMO (1999) and adopted for
this report. Montzka et al. (2000) extend the atmospheric record of CH3CCl3
to include the rapid decay over the last five years following cessation of emissions
and derive an OH lifetime of 6.3 years. The new information on the CH3CCl3
lifetime by Montzka et al. (2000) has not been incorporated into this report,
but it falls within the ±15% uncertainty for these lifetimes. If the
new value of 6.3 years were adopted, then the lifetime of CH4 would
increase to 9.2 yr, and all lifetimes, perturbation lifetimes, and GWPs for
gases controlled by tropospheric OH would be about 10% greater.